The invention relates to a multimode switched-mode power supply for operating in a first control mode when the output power supplied is lower than a predetermined value, and in a second control mode when the output power supplied is higher than the predetermined value.
NL-9201428 discloses a dual-mode switched-mode power supply comprising a transformer with a primary and a secondary winding. A series arrangement of the primary winding and a switch receives a mains voltage. Control pulses control the switch to stabilize an output voltage supplied by the secondary winding to a load. In a first control mode, when it is detected that the power supplied is lower than a predetermined value, the control pulses have a first fixed repetition frequency. In a second control mode, when it is detected that the power supplied is higher than a predetermined value, the control pulses have a second fixed repetition frequency which is higher than the first fixed repetition frequency. It is a drawback of the known dual-mode switched-mode power supply that mode transitions result in an undesirable ripple on the output voltage.
It is, inter alia, an object of the invention to improve the stabilization of the output voltage caused by the mode transitions while maintaining a high efficiency at a low output power.
To this end, a first aspect of the invention provides a multimode switched-mode power supply as claimed in claim 1. A second aspect of the invention provides a control circuit for use in a multimode switched-mode power supply as claimed in claim 5. Advantageous embodiments are defined in the dependent claims.
The multimode switched-mode power supply in accordance with the invention, operates in a first control mode when the output power supplied by the multimode switched-mode power supply (to one load or more loads) is lower than a predetermined value, and in a second control mode when the output power supplied is higher than this predetermined value. In the first control mode, the control circuit may control the frequency of the multimode switched-mode power supply to obtain a substantially constant peak value of the inductor current, independent of the output power supplied. The substantially constant peak value of the inductor current in the first control mode is substantially equal to a peak value of the inductor current at an instant when the second control mode is altered into the first control mode. The instant when the second control mode is altered into the first control mode is determined by the output power decreasing below the predetermined value. As the peak value of the inductor current is substantially constant during transitions between the control modes, the control behavior of the switched-mode power supply is not disturbed and the output voltage will be stabilized very well, even during these transitions. In the prior art, the peak value of the current in the primary winding jumps significantly when the repetition frequency of the control pulses changes during a mode transition (because, during a mode transition, the output power supplied is nearly constant). This peak current jump causes difficulties in the control loop of the switched-mode power supply with respect to stability and transient effects on the output voltage. The control loop will require some time to adapt to the new peak current value, and consequently, the stabilization of the output voltage will not be optimal.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.